The importance of incorporating the most effective arc suppression system into a reactive DC sputtering process stems from the fact that the conditions which cause an increase in the rate of formation of a film, increased power and increased flow of reactive gas into the chamber, are the same as those which increase the severity of arcing. Since the presence of arcing severely degrades any sputtering process, there is a maximum rate of film production that can be achieved at a given level of quality using an arc suppression system having a given effectiveness. An improvement in the arc suppression system therefore results in an increased production rate of a process, with resulting economic benefits.
An arc may take the form of an electrical discharge between the target, which is the cathode of the DC power supply, and some surrounding grounded conductor; for example, the conducting shield that is often located around the perimeter of the target. It may also be a so-called "unipolar" arc in which a discharge from the target to the plasma takes place. Once an arc is formed, it dissipates energy stored in the power supply as well as in the plasma.
It is necessary to control both the frequency and the energy of the arcs that appear during a sputtering process. Methods for doing this which are part of the prior art have involved refinements to the sputtering power supply. The present invention further enhances the arc-suppression performance of the power supply refinements, as will be made clear by the following discussion.
Prior art arc suppression systems may be grouped into those which employ some form of power supply interruption, in which the power supply is shut off for a period of time, or voltage reversal, in which the polarity of the power supply voltage (at reduced magnitude) is changed for a period of time. They may be further divided into groups which cause a periodically occurring interruption or reversal of fixed duration, and those which bring about the reversal or interruption only after the beginning of an arc has been detected.
Early sputtering power supplies were non-periodic voltage interruption devices which used an arc sensor built into the power supply. The power supply was designed to shut down for a period of time upon receipt of a signal from the sensor. For power supplies operating with standard line power, the time required for shutdown to occur is a few milliseconds, and the amount of energy dissipated in the arc during shutdown is the order of 5 to 10 Joules for a power supply that delivers between 5 and 10 kilowatts of power to the target, an amount of energy that can cause severe spattering of target material. The dissipated energy has been reduced in other supplies by converting the line frequency to a higher frequency and then using this higher frequency power supply as a source for the DC supply. The time required for shutoff as well as the dissipated energy are inversely proportional to the ratio of the new frequency to the former frequency, so that improvement by a factor of more than 250 can be achieved by changing from 60 Hz to 30 KHz. Power supplies that operate on line frequency are known as line supplies while those which use a higher frequency are called switching supplies.
A simple type of non-periodic voltage reversal system is one that has a built-in "ringing" response to a disturbance in its output. Such a system has reactive components which cause the output, when driven toward zero by the sudden current demand of the arc, to overshoot zero volts and become negative for a period of time long enough to suppress the arc.
The next level of advancement in sputtering power supplies was the introduction of periodic voltage reversals into the power supply output during the sputtering process. During the recurring period of reversal time, the conditions which cause arc formation are suppressed as follows. When sputtering is underway, the target is charged negatively, so that positive ions are attracted to it, causing the desired sputtering. The flow of ions causes positive charge to be deposited not only on conducting portions of the target but also on certain areas of the target that are covered by an insulating layer of sputtered material. Arcs are initiated by electrical breakdown of this insulating layer. During sputtering, as the outer surface of this layer receives a continual bombardment of positive sputtering gas ions, a buildup of positive charge on this surface occurs. This results in a electric field within the layer that increases with time. When the charge density on the surface becomes great enough to cause a field within the insulating layer that exceeds the electrical breakdown strength of the insulating material, a discharge through the layer occurs. The resulting explosive release of material from the discharge site provides a conducting path for a subsequent arc between the target and a nearby conductor or between the target and the plasma.
When a reversed (positive) voltage is applied to the target, the voltage of the outer surface of the insulator becomes positive with respect to ground, causing electrons to flow from the adjacent plasma to the charged surface. This electron flow neutralizes some of the positive charge so that the field within the insulating layer is reduced. If the period of time of charge buildup is kept short enough to prevent breakdown of the insulator during the sputtering part of the cycle, and if conditions are proper during the period of reversal, the arcing can be greatly curtailed.
A straightforward method for achieving voltage reversal is to superimpose upon the target supply a voltage oscillating at a radio frequency (RF) such as 450 KHz, where the peak-to-peak amplitude of the RF signal is sufficient to cause the voltage on the target to be reversed during part of the positive RF cycle. This method has proven to be effective, but it requires the introduction of high levels of RF power which often is not desirable.
A series of controlled voltage reversal systems for arc suppression has been developed especially for DC reactive sputtering by Advanced Energy Systems of Fort Collins, Colo. They are designed to operate with a switching supply and are identified by the trade name SPARC.TM. or SPARCLE.TM.. These systems, unlike the RF voltage reversal technique do not require that a significant amount of additional power be dissipated in the coating chamber when they are in use.
The SPARC-LE.TM. systems employ a unit which is inserted between the main power supply and the sputtering target. In an externally triggered mode, in which the systems are non-periodic, a sensor located in the additional unit detects the formation of an arc by monitoring the power supply output. When the arc is detected, the current from the main power supply is switched by the new unit into an inductor which has been preloaded with the same current that the power supply had previously been delivering to the target. There is therefore no change in the current demand on the power supply so that disruptive transients on the power supply output are eliminated. At the same time that the power supply current is switched to the inductor, a small positive voltage in the order of 50 Volts, which is generated by the new unit, is switched onto the target and takes the place of the former negative voltage. The reversal of potential combined with the characteristics that have been designed into the SPARC-LE.TM. unit cause the arc to be quenched.
In the self-triggered mode, the above described switching process involving voltage reversals at the sputtering target takes place periodically and does not require that the formation of an arc be sensed. In this mode, the system becomes a periodic reversal system whose principle of operation has been discussed previously; however it incorporates additional refinements. The durations of the periods of negative and positive voltages can be varied to meet the requirements of a particular process. Typically, the cathode is biased at its high negative operating voltage for 45 microseconds and at a low positive voltage for 5 microseconds, so that the process repeats every 50 microseconds. The period of time over which the negative voltage is applied is chosen to be short enough to prevent areas of the target which are covered by insulator from acquiring enough charge to cause breakdown. Therefore arcing is averted. During the voltage reversal, the charged areas are discharged by electrons in the plasma surrounding the target. If conditions are sufficiently benign, the discharge is complete and the cycle continues indefinitely with potential arc sites being alternately charged and discharged without acquiring enough charge to cause an arc.
The SPARC-LE.TM. systems can operate in a self triggered mode and an externally triggered mode simultaneously. During such operation they provide the aforementioned periodic waveform to the sputtering target while monitoring the power supply output to detect the beginning of an arc during the period of negative voltage. If an arc is detected, voltage reversal is instituted and the system behaves as previously described for the externally triggered mode. After the arc is quenched, periodic behavior resumes. By the action described, the SPARC-LE.TM. system greatly reduces the frequency of arcing in many sputtering processes.
Any voltage interruption or reversal arc suppression system utilizes the principle that when the target voltage is reversed (i.e. made positive) or caused to become zero, the plasma adjacent to the target in the sputtering chamber must deposit a negative charge on each potential arc site, which charge at least partially neutralizes the positive charge that was deposited by the ions left during the previous sputtering period. As the sputtering rate of a particular configuration is increased, the rate of charge deposition is increased, and if the rate of arcing is not to increase, the plasma must provide a correspondingly greater flow of neutralizing electrons. The plasma generated by the sputtering target and its power supply is limited in both the rate at which it can supply electrons and the total number of electrons that it can supply. It is therefore necessary, if the rate of arcing is to be held constant and the sputtering rate is to increase, that a means for increasing the rate of electron delivery and the total supply of electrons be provided.
It is therefore the object of this invention to generate an intense plasma in contact with the sputtering target, the plasma having sufficient density, electron temperature and volume to fully discharge potential arc sites on the target during periods of voltage interruption or reversal provided by the sputtering power supply for the purpose of arc suppression, and by means of the action of this plasma to substantially reduce the frequency and energy of arcing of a reactive sputtering process which employs a given arc-suppression system.
It is a further object of this invention to provide a means for enhancing arc suppression in a reactive sputtering process wherein said means which can be controlled separately from the power supply and thereby to gain increased flexibility in choosing operating conditions for a given process.